Infinite sequences via Lie algebra actions for oligomorphic groups

Abstract: Many integer sequences arise as numbers of $G$-orbits on $\binom{X}{n}$ as $n$ varies, for a permutation group $G\subseteq \operatorname{Sym}(X)$. For finite $X$, Stanley proved that these finite sequences increase towards the middle using an action of the Lie algebra $\mathfrak{sl}2(\mathbb{C})$. For infinite sets $X$, and hence infinite sequences, Cameron provided an argument for monotonicity. He first identifies orbits with a vector space basis of a certain commutative $k$-algebra $\mathsf{H}{G,X}^{\star}$, called the orbit algebra. He then considers the operator, which forms the product with the constant $1$-function on $X$, and proves its injectivity. In this paper we generalize Stanley's approach to oligomorphic groups, and in particular extend Cameron's operator to a full $\mathfrak{sl}2(\mathbb{C})$-action on $\mathsf{H}{G,X}^{\star}$. We define for every oligomorphic permutation group $G\subseteq \operatorname{Sym}(X)$ the $X$-th tensor power $(k^r)^{\otimes X}$, generalizing work of Entova-Aizenbud. We show that this space carries natural commuting actions of $G$ and the Lie algebra $\mathfrak{gl}r(k)$, the latter depending on a Harman--Snowden measure $μ$ on $G$. We then show that $\mathsf{H}{G,X}^{\star}\subseteq (\mathbb{C}^2)^{\otimes X}$ can be decomposed into a direct sum of $\mathfrak{sl}_2(\mathbb{C})$-Verma modules, which gives monotonicity. We explain how our approach applies to Fibonacci numbers, Tribonacci numbers, etc. by constructing measures on products with $(\mathbb{Q},<)$.